Controlled gap states for liquid crystal displays

ABSTRACT

The present invention relates to a bistable matrix-addressable display element comprising a substrate, a bistable electrically modulated imaging layer having a reflection maximum, at least one conductor, and at least one field-spreading layer between said bistable electrically modulated imaging layer and said at least one conductor, wherein said field-spreading layer has a sheet resistance (SER) of less than 10 9  Ohms per square and a method of imaging the display comprising identifying an area to be updated of said bistable matrix-addressable display element, wherein said area to be updated comprises rows of pixels; and applying a sequence of drive signals having a 4-phase approach to image said bistable matrix-addressable display element, which may be characterized as a planar reset, left-slope selection method.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly assigned, co-pending U.S. patentapplications:

Ser. No. ______ by Stanley W. Stephenson, III (Docket 89296) filed ofeven date herewith entitled “Field Blooming Color Filter Layer ForDisplays”;

Ser. No. ______ by Burberry et al. (Docket 89357) filed of even dateherewith entitled “Reflective Layer Field Blooming Layer For LCDisplay”; and

Ser. No. ______ by Burberry et al. (Docket 90600) filed of even dateherewith entitled “Conductive Layer To Reduce Drive Voltage InDisplays”, incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to bistable displays having one or morefield-spreading layers with improved contrast, which in turn produce ahigher quality display, especially when a particular sequence of drivesignals is applied.

BACKGROUND OF THE INVENTION

Currently, information is displayed using assembled sheets of papercarrying permanent inks or displayed on electronically modulatedsurfaces such as cathode ray displays or liquid crystal displays. Othersheet materials can carry magnetically written areas to carry ticketingor financial information, however magnetically written data is notvisible.

A structure is disclosed in PCT/WO 97/04398, entitled “Electronic BookWith Multiple Display Pages” which is a thorough recitation of the artof thin, electronically written display technologies. Disclosed is theassembling of multiple display sheets that are bound into a “book”, eachsheet can be individually addressed. The patent recites prior art informing thin, electronically written pages, including flexible sheets,image modulating material formed from a bi-stable liquid crystal system,and thin metallic conductor lines on each page.

Fabrication of flexible, electronically written display sheets aredisclosed in U.S. Pat. No. 4,435,047. A first sheet has transparent ITOconductive areas and a second sheet has electrically conductive inksprinted on display areas. The sheets can be glass, but in practice havebeen formed of Mylar polyester. A dispersion of liquid crystal materialin a binder is coated on the first sheet, and the second sheet is bondedto the liquid crystal material. Electrical potential applied to opposingconductive areas operate on the liquid crystal material to exposedisplay areas. The display uses nematic liquid crystal material thatceases to present an image when de-energized.

U.S. Pat. No. 5,437,811 discloses a light-modulating cell having apolymer dispersed chiral nematic liquid crystal. The chiral nematicliquid crystal has the property of being driven between a planar statereflecting a specific visible wavelength of light and a light scatteringfocal conic state. The structure has the capacity of maintaining one ofthe given states in the absence of an electric field.

U.S. Pat. No. 3,816,786 discloses a layer of encapsulated cholestericliquid crystal responsive to an electric field. The conductors in thepatent can be transparent or non-transparent and formed of variousmetals or graphite. It is disclosed that one conductor must be lightabsorbing and it is suggested that the light absorbing conductor beprepared from paints containing conductive material such as carbon.

U.S. Pat. No. 5,289,301 discusses forming a conductive layer over aliquid crystal coating to form a second conductor. The description ofthe preferred embodiment discloses Indium-Tin-Oxide (ITO) over a liquidcrystal dispersion to create a transparent conductor.

Cholesteric materials require one of the two conductors to be lightabsorbing and conductive. Materials have been proposed for theapplication including carbon or metal oxides to create a black andconductive surface for polymer dispersed cholesteric liquid crystalmaterials. Such coatings often backscatter light. Moreover, becausethere is inactive material between the conductors, it would be desirableto maximize the use of the inactive material.

U.S. Pat. No. 6,639,637 describes an opaque field-spreading layer fordispersed liquid crystal coatings that allow the electrical switching ofmaterial in the usually inactive regions (gaps) between the secondconductors. U.S. Pat. No. 6,707,517 describes a transparentfield-spreading layer for dispersed liquid crystal coatings that allowthe electrical switching of material in gaps between the firstconductors. Although field-spreading layers and state switching of gapmaterial was demonstrated, the sequence of signals necessary forswitching the gaps in passive matrix displays into the most desiredpattern of light and dark states was not recognized.

There are many means for driving cholesteric liquid crystals in apassive matrix. U.S. Pat. No. 5,644,330 teaches how to utilize the rightslope of the electro-optical curves of chiral nematic liquid crystal. Inaddition, a clearing voltage is described that initializes the panel'sreflective state. This clearing voltage can be sufficient to set thedisplay into the focal conic texture. If the clearing voltage is highenough, the panel can be initialized into the stable planar texture.This prior art teaches that clearing the display before rewriting itassists in the removal of residual previous image. These drive methods,also referred to as drive schemes, can be described as conventionalfocal conic rest, right-slope select and conventional planar rest,right-slope select methods respectively.

Another drive method for cholesteric liquid crystals is pulsecumulative, as described in U.S. Pat. No. 6,133,895. In this method, aseries of voltage pulses are applied to the display at a frequencyapproximately 60 Hz. A series of 6 or 7 voltage pulses cumulativelychange the reflectance state of the pixel in an array of pixels. Methodsof this type can be characterized as pulse-accumulation right-slopeselect.

Dynamic drive methods are also well known in the art of driving chiralnematic liquid crystal displays. U.S. Pat. No. 5,748,277 describes a 3phase dynamic drive scheme where the fast transition from homeotropic tothe transient planar texture is leveraged for high speed chiral nematicliquid crystal writing. SID 2001 Digest, pp. 882-885 “Simple DriveScheme for Bistable Cholesteric LCDs” (Rybalochka, et. al) teaches asimple dynamic drive method utilizing a pseudo 3-phase that requiresonly 2 voltage levels for row and column drivers. This method is knownas the “U/√2” driving method. The symbol “√” is understood by those ofskill in the art to indicate the square root. For example, √2 representsthe square root of 2.

U.S. Patent Application 2005/0024307 A1 discloses yet another drivingmethod for chiral nematic liquid crystal displays that follows a moreconventional approach. The use of a high voltage planar reset pulse tothe entire panel prior to writing the display, as well as use of theleft slope of the electro-optic curve for cholesteric liquid crystaldisplays. This drive method applies a high voltage pulse followed by alow voltage series of pulses to specific pixels that are to betransitioned from the stable planar texture (established in the resetpulse) to focal conic. During the selection phase, the voltage acrossthe pixels to be transitioned to the focal conic texture is greater thanthe voltage across the pixels that are to remain in the stable planartexture, which is the distinguishing characteristic of a left-slopedrive method. Therefore, this method can be described as a planar reset,left-slope selection method.

It is the aim of this invention to describe image displays havingfield-spreading layers and the drive methods desired to image thedisplays having field-spreading layers to their best advantage.

Problem to be Solved

There is a need for bistable displays with improved contrast, which inturn produces a higher quality display.

SUMMARY OF THE INVENTION

The present invention relates to a bistable matrix-addressable displayelement comprising a substrate, a bistable electrically modulatedimaging layer having a reflection maximum, at least one conductor, andat least one field-spreading layer between the bistable electricallymodulated imaging layer and the at least one conductor, wherein thefield-spreading layer has a sheet resistance (SER) of from 10⁹ to 10⁶Ohms per square. The present invention also relates to a method ofimaging the display comprising identifying an area to be updated of thebistable matrix-addressable display element, wherein the area to beupdated comprises rows of pixels; and applying a sequence of drivesignals having a 4-phase approach to image the bistablematrix-addressable display element, wherein the 4-phase approachcomprises in phase 1, applying a first pixel voltage across the pixelsof the area to be updated such that the critical voltage is reached,holding the first pixel voltage until a homeotropic texture is reached;in phase 2, setting a second pixel voltage to allow the homeotropictexture to relax into a stable planar texture; in phase 3, selecting onerow of pixels of the rows of pixels of the area to be updated, updatingthis one row of pixels by addressing, wherein addressing comprisesapplying a third pixel voltage, capable of switching the pixels from thestable planar texture to the non-reflective focal conic texture, acrossthe pixels to produce switched pixels; and applying a fourth pixelvoltage, incapable of switching the pixels from the stable planartexture to the non-reflective focal conic texture, to produce unswitchedpixels to remain in the stable planar texture; and repeating theaddressing until the rows of pixels of the area to be updated have beenaddressed; and, in phase 4, removing the first pixel voltage, the secondpixel voltage, the third pixel voltage, and the fourth pixel voltagefrom the area of the bistable matrix-addressable display element to beupdated.

Advantageous Effect of the Invention

The present invention includes several advantages, not all of which areincorporated in a single embodiment. The present invention provides adisplay in which the gaps between conductive electrodes are imaged asdesired in light and dark areas, resulting in improved contrast. Whenutilized in combination with a particular driver, contrast is enhancedfurther. The field-spreading also results in a display which operateswith reduced voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of a passive matrix display structurecross section.

FIG. 2 illustrates characteristic curves.

FIG. 3 illustrates the desired and undesirable gap reflectionconditions.

FIG. 4 illustrates another embodiment of a passive matrix display crosssection.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a display element comprised of asubstrate, an electrically modulated imaging layer, a conductor disposedbetween the substrate and the imaging layer, and a field-spreadinglayer. Optionally, the display may include a second conductor on theside of the imaging layer opposite the first conductor. The inventionalso relates to a method of imaging the inventive display element.

The present invention utilizes a partially conductive field-spreadinglayer. The conductive character of the field-spreading layer may becharacterized as incapable of shorting or directly carrying a field. Thepartially conductive field-spreading layer typically demonstrates asheet resistance, also referred to as surface electrical resistance(SER), of less than 10⁹ Ohms/square, most typically in the range of from10⁹ to 10⁶ Ohms per square. The field spreading character of the layeris provided by electronic conductors contained in the layer.

In the present invention, the first conductor, typically disposedbetween the substrate and the imaging layer, is patterned withnon-conductive spaces between adjacent conductive areas. The conductiveareas of the first conductor are referred to as columns and thenon-conductive spaces between adjacent columns are referred to as columngaps. The second conductor also is patterned with non-conductive spacesbetween adjacent conductive areas. The conductive areas of the secondconductor are referred to as rows and the non-conductive spaces betweenadjacent rows are referred to as row gaps. The field-spreading layer hasa sheet resistance, also referred to as surface electrical resistance(SER), capable of spreading an applied field across the width ofadjacent column or row gaps.

To image the display, electrical signals corresponding to a reset phaseare followed by a relaxation phase and a selection phase and where, inthe reset phase followed by the relaxation phase, the rows and columnsare energized with pulse sequences that leaves the display in a fullyreflective state and where, in the selection phase, each row isaddressed such that pixels that are to be switched from the reflectivetexture to the non-reflective texture receive a pixel voltage pulse orset of pulses across them in the range between V2 and V3 while pixelsthat are to remain in the stable reflective state receive a pulse or setof pulses in the range between V0 and V1 that have negligible effect onthe final texture of the pixel. The row may preferably be sequentiallyaddressed. Exemplary values for V0, V1, V2, V3 and V4 are shown in FIG.2. For purposes of the present invention, V4 represents a thresholdvoltage, frequently referred to as the critical voltage, related to theelectro-optical characteristic curve. V0 represents the voltage atground, where voltage equals 0 (zero) volts. V1 represents the highestvoltage pulse applied which does not significantly change or disturb theplanar texture of the imaging layer. For purposes of the presentinvention, a 1% or greater change is considered significant. V2represents the lowest voltage that can be applied to move to within 1%of pure, that is, maximum, focal conic texture. In FIG. 2, this would bethe lowest point on the electro-optical characteristic curve. V3represents the highest voltage which results in not significant changeor disturbance from focal conic texture. V4 represents the minimumvoltage required to convert the focal conic texture back to the planartexture, that is, the minimum voltage required to drive the liquidcrystal to a homeotropic state. The voltage reduction achieved by thepresent invention over the prior art relates to the value of V4. Thesevoltages are also referred to as pixel voltages. Pixel voltages aredefined herein as the difference between the row voltage and the columnvoltage.

Electronic conductors such as conjugated conducting polymers, conductingcarbon particles, crystalline semiconductor particles, amorphoussemiconductive fibrils, and continuous conductive metal orsemiconducting thin films can be used in this field-spreading layer.Useful conductors may also include ionic conductors, provided that theionic conductor can be substantially confined to the field-spreadinglayer. Of the various types of electronic conductors, electronicallyconductive metal-containing particles, such as semiconducting metaloxides, and electronically conductive polymers, such as, substituted orunsubstituted polythiophenes, substituted or unsubstituted polypyrroles,and substituted or unsubstituted polyanilines are particularly effectivefor the present invention.

Conductive metal-containing particles, which may be used in the presentinvention include conductive crystalline inorganic oxides, conductivemetal antimonates, and conductive inorganic non-oxides. Crystallineinorganic oxides may be chosen from zinc oxide, titania, tin oxide,alumina, indium oxide, silica, magnesia, barium oxide, molybdenum oxide,tungsten oxide, and vanadium oxide or composite oxides thereof, asdescribed in, e.g., U.S. Pat. Nos. 4,275,103, 4,394,441, 4,416,963,4,418,141, 4,431,764, 4,495,276, 4,571,361, 4,999,276 and 5,122,445, allincorporated herein by reference. The conductive crystalline inorganicoxides may contain a “dopant” in the range from 0.01 to 30 mole percent,preferred dopants being aluminum or indium for zinc oxide, niobium ortantalum for titania, and antimony, niobium or halogens for tin oxide.Alternatively, the conductivity can be enhanced by formation of oxygendefects by methods well known in the art. The use of antimony-doped tinoxide at an antimony doping level of at least 8 atom percent and havingan X-ray crystallite size less than 100 Å and an average equivalentspherical diameter less than 15 nm but no less than the X-raycrystallite size as taught in U.S. Pat. No. 5,484,694 is specificallycontemplated and incorporated herein by reference.

Particularly useful electronically conductive metal-containingparticles, which may be used in the field-spreading layer, includeacicular-doped metal oxides, acicular metal oxide particles, andacicular metal oxides containing oxygen deficiencies. In this category,acicular doped tin oxide particles, particularly acicular antimony-dopedtin oxide particles, acicular niobium-doped titanium dioxide particles,and the like are preferred because of their availability. The aforesaidacicular conductive particles preferably have a cross-sectional diameterless than or equal to 0.02 mm and an aspect ratio greater than or equalto 5:1. Some of these acicular conductive particles, useful for thepresent invention, are described in U.S. Pat. Nos. 5,719,016, 5,731,119,5,939,243, incorporated herein by reference, and references therein.

The invention is also applicable where the conductive agent comprises aconductive “amorphous” gel such as vanadium oxide gel comprised ofvanadium oxide ribbons or fibers. Such vanadium oxide gels may beprepared by any variety of methods, including but not specificallylimited to melt quenching as described in U.S. Pat. No. 4,203,769,incorporated herein by reference, ion exchange as described in DE4,125,758, incorporated herein by reference, or hydrolysis of a vanadiumoxoalkoxide as claimed in WO 93/24584, incorporated herein by reference.The vanadium oxide gel is preferably doped with silver to enhanceconductivity. Other methods of preparing vanadium oxide gels which arewell known in the literature include reaction of vanadium or vanadiumpentoxide with hydrogen peroxide and hydrolysis of VO2 OAc or vanadiumoxychloride.

Conductive metal antimonates suitable for use in accordance with theinvention include those as disclosed in, U.S. Pat. Nos. 5,368,995 and5,457,013, for example, incorporated herein by reference. Preferredconductive metal antimonates have a rutile or rutile-relatedcrystallographic structures and may be represented as M+2 Sb+52 O6(where M+2=Zn+2, Ni+2, Mg+2,Fe+2, Cu+2, Mn+2, Co+2) or M+3 Sb+5 O4(where M+3=In +3, Al+3, Sc+3, Cr+3, Fe+3). Several colloidal conductivemetal antimonate dispersions are commercially available from NissanChemical Company in the form of aqueous or organic dispersions.Alternatively, U.S. Pat. Nos. 4,169,104 and 4,110,247, both incorporatedherein by reference, teach a method for preparing M+2 Sb+52 O6 bytreating an aqueous solution of potassium antimonate with an aqueoussolution of an appropriate metal salt (e.g., chloride, nitrate, sulfate)to form a gelatinous precipitate of the corresponding insoluble hydratewhich may be converted to a conductive metal antimonate by suitabletreatment.

Conductive inorganic non-oxides suitable for use as conductive particlesin the present invention include metal nitrides, metal borides and metalsilicides, which may be acicular or non-acicular in shape. Examples ofthese inorganic non-oxides include titanium nitride, titanium boride,titanium carbide, niobium boride, tungsten carbide, lanthanum boride,zirconium boride, molybdenum boride and the like. Examples of conductivecarbon particles, include carbon black and carbon fibrils or nanotubeswith single walled or multi-walled morphology. Example of such suitableconductive carbon particles can be found in U.S. Pat. No. 5,576,162,incorporated herein by reference, and references therein.

Suitable electrically conductive polymers that are preferred forincorporation in the antistatic layer used with the invention arespecifically electronically conducting polymers, such as thoseillustrated in U.S. Pat. Nos. 6,025,119, 6,060,229, 6,077,655,6,096,491, 6,124,083, 6,162,596, 6,187,522, and 6,190,846, allincorporated herein by reference. These electronically conductivepolymers include substituted or unsubstituted aniline-containingpolymers (as disclosed in U.S. Pat. Nos. 5,716,550, 5,093,439 and4,070,189, incorporated herein by reference), substituted orunsubstituted thiophene-containing polymers (as disclosed in U.S. Pat.Nos. 5,300,575, 5,312,681, 5,354,613, 5,370,981, 5,372,924, 5,391,472,5,403,467, 5,443,944, 5,575,898, 4,987,042 and 4,731,408, allincorporated herein by reference), substituted or unsubstitutedpyrrole-containing polymers (as disclosed in U.S. Pat. Nos. 5,665,498and 5,674,654, both incorporated herein by reference), andpoly(isothianaphthene) or derivatives thereof. These conducting polymersmay be soluble or dispersible in organic solvents or water or mixturesthereof. Preferred conducting polymers for the present invention includepolypyrrole styrene sulfonate (referred to as polypyrrole/poly (styrenesulfonic acid) in U.S. Pat. No. 5,674,654, incorporated herein byreference), 3,4-dialkoxy substituted polypyrrole styrene sulfonate, and3,4-dialkoxy substituted polythiophene styrene sulfonate because oftheir color. The most preferred substituted electronically conductivepolymers include poly(3,4-ethylene dioxythiophene styrene sulfonate),such as Baytron â P supplied by Bayer Corporation, for its apparentavailability in relatively large quantity.

Other materials may be included in the field-spreading layer. Forexample, the field-spreading layer may also contain a colorant toproduce a color contrast field-spreading layer. Preferably, the colorantwill provide a contrasting color to the reflection maximum of the nearbybistable electrically modulated imaging layer.

The field-spreading layers may be applied to the support or other layersof the display by any method known by those of skill in the art to forma layer. Some exemplary methods may include screen printing, hoppercoating, gravure printing, lithographic and photolithographic printing,spraying, vapor depositing, dip coating, rod coating, blade coating, airknife coating, gravure coating and reverse roll coating, extrusioncoating, slide coating, curtain coating, and the like. Thefield-spreading layer may be applied simultaneously with other layers,sequentially to other layers, or in any combination of simultaneous andsequential application.

FIG. 4 is a sectional view of an exemplary display sheet according tothe present invention having polymer dispersed cholesteric layer 30 inaccordance with the present invention. A transparent field-spreadinglayer 24 is disposed between first conductors 20 and polymer dispersedcholesteric layer 30. The transparent organic conductor can be Baytron Bpolythiophene suspension from Agfa-Gevaert N. V. of Morsel, Belgium. Foran experimental coating, field-spreading layer 24 can be 1.0 weightpercent deionized gelatin and 1.0 weight percent sub-micron(nanoparticle) polythiophene coated over first conductors 20 at a 25micron wet thickness. The dried transparent field-spreading layer 24will be approximately 0.4 microns thick. The resulting transparentfield-spreading layer 24 is functionally transparent and has a sheetresistance, also referred to as surface electrical resistance (SER), ofover one mega-Ohms per square. The resulting coating is functionallynon-conductive compared to adjacent first conductors 20. Transparentfield-spreading layer 24 can activate cholesteric liquid crystalmaterial past the edge of a field-carrying electrode. In making sheet10, the sheet can be in the form of a web that is sequentially movedthrough one or more stations which sequentially or simultaneouslydeposits the state changing layer 30 or transparent field-spreadinglayer 24.

In one embodiment, at least one imageable layer is applied to thesupport. The imageable layer can contain an electrically imageablematerial. The electrically imageable material can be light emitting orlight modulating. Light emitting materials can be inorganic or organicin nature. Particularly preferred are organic light emitting diodes(OLED) or polymeric light emitting diodes (PLED). The light modulatingmaterial can be reflective or transmissive. Light modulating materialscan be electrochemical, electrophoretic, such as Gyricon particles,electrochromic, or liquid crystals. The liquid crystalline material canbe twisted nematic (TN), super-twisted nematic (STN), ferroelectric,magnetic, or chiral nematic liquid crystals. Especially preferred arechiral nematic liquid crystals. The chiral nematic liquid crystals canbe polymer dispersed liquid crystals (PDLC). Structures having stackedimaging layers or multiple support layers, however, are optional forproviding additional advantages in some case.

In a preferred embodiment, the electrically imageable material can beaddressed with an applied electric field and then retain its image afterthe electric field is removed, a property typically referred to as“bistable”. Preferred bistable electrically modulated imaging layers arefield or voltage driven switching layer. Particularly suitableelectrically imageable materials that exhibit “bistability” areelectrochemical, electrophoretic, such as Gyricon particles,electrochromic, magnetic, or chiral nematic liquid crystals. Especiallypreferred are chiral nematic liquid crystals. The chiral nematic liquidcrystals can be polymer dispersed liquid crystals (PDLC).

The electrically modulated material may also be a printable, conductiveink having an arrangement of particles or microscopic containers ormicrocapsules. Each microcapsule contains an electrophoretic compositionof a fluid, such as a dielectric or emulsion fluid, and a suspension ofcolored or charged particles or colloidal material. The diameter of themicrocapsules typically ranges from about 30 to about 300 microns.According to one practice, the particles visually contrast with thedielectric fluid. According to another example, the electricallymodulated material may include rotatable balls that can rotate to exposea different colored surface area, and which can migrate between aforward viewing position and/or a rear nonviewing position, such asgyricon. Specifically, gyricon is a material comprised of twistingrotating elements contained in liquid filled spherical cavities andembedded in an elastomer medium. The rotating elements may be made toexhibit changes in optical properties by the imposition of an externalelectric field. Upon application of an electric field of a givenpolarity, one segment of a rotating element rotates toward, and isvisible by an observer of the display. Application of an electric fieldof opposite polarity, causes the element to rotate and expose a second,different segment to the observer. A gyricon display maintains a givenconfiguration until an electric field is actively applied to the displayassembly. Gyricon particles typically have a diameter of about 100microns. Gyricon materials are disclosed in U.S. Pat. No. 6,147,791,U.S. Pat. No. 4,126,854 and U.S. Pat. No. 6,055,091, the contents ofwhich are herein incorporated by reference.

According to one practice, the microcapsules may be filled withelectrically charged white particles in a black or colored dye. Examplesof electrically modulated material and methods of fabricating assembliescapable of controlling or effecting the orientation of the ink suitablefor use with the present invention are set forth in International PatentApplication Publication Number WO 98/41899, International PatentApplication Publication Number WO 98/19208, International PatentApplication Publication Number WO 98/03896, and International PatentApplication Publication Number WO 98/41898, the contents of which areherein incorporated by reference.

The electrically modulated material may also include material disclosedin U.S. Pat. No. 6,025,896, the contents of which are incorporatedherein by reference. This material comprises charged particles in aliquid dispersion medium encapsulated in a large number ofmicrocapsules. The charged particles can have different types of colorand charge polarity. For example white positively charged particles canbe employed along with black negatively charged particles. The describedmicrocapsules are disposed between a pair of electrodes, such that adesired image is formed and displayed by the material by varying thedispersion state of the charged particles. The dispersion state of thecharged particles is varied through a controlled electric field appliedto the electrically modulated material. According to a preferredembodiment, the particle diameters of the microcapsules are betweenabout 5 microns and about 200 microns, and the particle diameters of thecharged particles are between about one-thousandth and one-fifth thesize of the particle diameters of the microcapsules.

Further, the electrically modulated material may include a thermochromicmaterial. A thermochromic material is capable of changing its statealternately between transparent and opaque upon the application of heat.In this manner, a thermochromic imaging material develops images throughthe application of heat at specific pixel locations in order to form animage. The thermochromic imaging material retains a particular imageuntil heat is again applied to the material. Since the rewritablematerial is transparent, UV fluorescent printings, designs and patternsunderneath can be seen through.

The electrically modulated material may also include surface stabilizedferrroelectric liquid crystals (SSFLC). Surface stabilized ferroelectricliquid crystals confining ferroelectric liquid crystal material betweenclosely spaced glass plates to suppress the natural helix configurationof the crystals. The cells switch rapidly between two opticallydistinct, stable states simply by alternating the sign of an appliedelectric field.

Magnetic particles suspended in an emulsion comprise an additionalimaging material suitable for use with the present invention.Application of a magnetic force alters pixels formed with the magneticparticles in order to create, update or change human and/or machinereadable indicia. Those skilled in the art will recognize that a varietyof bistable nonvolatile imaging materials are available and may beimplemented in the present invention.

The electrically modulated material may also be configured as a singlecolor, such as black, white or clear, and may be fluorescent,iridescent, bioluminescent, incandescent, ultraviolet, infrared, or mayinclude a wavelength specific radiation absorbing or emitting material.There may be multiple layers of electrically modulated material.Different layers or regions of the electrically modulated materialdisplay material may have different properties or colors. Moreover, thecharacteristics of the various layers may be different from each other.For example, one layer can be used to view or display information in thevisible light range, while a second layer responds to or emitsultraviolet light. The nonvisible layers may alternatively beconstructed of non-electrically modulated material based materials thathave the previously listed radiation absorbing or emittingcharacteristics. The electrically modulated material employed inconnection with the present invention preferably has the characteristicthat it does not require power to maintain display of indicia.

Most preferred is a support bearing a conventional polymer dispersedlight modulating material. The liquid crystal (LC) is used as an opticalswitch. The supports are usually manufactured with transparent,conductive electrodes, in which electrical “driving” signals arecoupled. The driving signals induce an electric field which can cause aphase change or state change in the liquid crystal material, the liquidcrystal exhibiting different light reflecting characteristics accordingto its phase and/or state

As used herein, a “liquid crystal display” (LCD) is a type of flat paneldisplay used in various electronic devices. At a minimum, a liquidcrystal display (LCD) comprises a substrate, at least one conductivelayer and a liquid crystal layer. Liquid crystal displays may alsocomprise two sheets of polarizing material with a liquid crystalsolution between the polarizing sheets. The sheets of polarizingmaterial may comprise a substrate of glass or transparent plastic. Theliquid crystal display may also include functional layers. In oneembodiment of an LCD 10, illustrated in FIG. 1, a transparent,multilayer flexible support 15 is coated with a first conductive layer20, which may be patterned, onto which is coated the light modulatingliquid crystal layer 30. A field-spreading layer 32 is coated on thelight modulating layer and a second patterned conductive layer 40 isapplied.

In another embodiment of a liquid crystal display 10, illustrated inFIG. 4, a transparent, multilayer flexible support 15 is coated with afirst conductive layer 20, which may be patterned, onto which is coateda field-spreading layer 24 followed by a light modulating liquid crystallayer 30. Layer 32 can be a color contrasting layer or a field-spreadinglayer which is applied on the light modulating layer. A second patternedconductive layer 40 can then be applied.

The liquid crystal (LC) is used as an optical switch. The substrates areusually manufactured with transparent, conductive electrodes, in whichelectrical “driving” signals are coupled. The driving signals induce anelectric field which can cause a phase change or state change in theliquid crystal material, the liquid crystal exhibiting different lightreflecting characteristics according to its phase and/or state.

Liquid crystals can be nematic (N), chiral nematic (N*), or smectic,depending upon the arrangement of the molecules in the mesophase. Chiralnematic liquid crystal (N*LC) displays are typically reflective, thatis, no backlight is needed, and can function without the use ofpolarizing films or a color filter.

Chiral nematic liquid crystal refers to the type of liquid crystalhaving finer pitch than that of twisted nematic and super-twistednematic used in commonly encountered liquid crystal devices. Chiralnematic liquid crystals are so named because such liquid crystalformulations are commonly obtained by adding chiral agents to hostnematic liquid crystals. Chiral nematic liquid crystals may be used toproduce bistable or multi-stable displays. These devices havesignificantly reduced power consumption due to their nonvolatile“memory” characteristic. Since such displays do not require a continuousdriving circuit to maintain an image, they consume significantly reducedpower. Chiral nematic displays are bistable in the absence of a field;the two stable textures are the reflective planar texture and the weaklyscattering focal conic texture. In the planar texture, the helical axesof the chiral nematic liquid crystal molecules are substantiallyperpendicular to the substrate upon which the liquid crystal isdisposed. In the focal conic state the helical axes of the liquidcrystal molecules are generally randomly oriented. Adjusting theconcentration of chiral dopants in the chiral nematic material modulatesthe pitch length of the mesophase and, thus, the wavelength of radiationreflected. Chiral nematic materials that reflect infrared radiation andultraviolet have been used for purposes of scientific study. Commercialdisplays are most often fabricated from chiral nematic materials thatreflect visible light. Some known liquid crystal display devices includechemically etched, transparent, conductive layers overlying a glasssubstrate as described in U.S. Pat. No. 5,667,853, incorporated hereinby reference.

In one embodiment, a chiral nematic liquid crystal composition may bedispersed in a continuous matrix. Such materials are referred to as“polymer dispersed liquid crystal” materials or “PDLC” materials. Suchmaterials can be made by a variety of methods. For example, Doane et al.(Applied Physics Letters, 48, 269 (1986)) disclose a PDLC comprisingapproximately 0.4 μm droplets of nematic liquid crystal 5CB in a polymerbinder. A phase separation method is used for preparing the PDLC. Asolution containing monomer and liquid crystal is filled in a displaycell and the material is then polymerized. Upon polymerization theliquid crystal becomes immiscible and nucleates to form droplets. Westet al. (Applied Physics Letters 63, 1471 (1993)) disclose a PDLCcomprising a chiral nematic mixture in a polymer binder. Once again aphase separation method is used for preparing the PDLC. The liquidcrystal material and polymer (a hydroxy functionalizedpolymethylmethacrylate) along with a crosslinker for the polymer aredissolved in a common organic solvent toluene and coated on an indiumtin oxide (ITO) substrate. A dispersion of the liquid crystal materialin the polymer binder is formed upon evaporation of toluene at hightemperature. The phase separation methods of Doane et al. and West etal. require the use of organic solvents that may be objectionable incertain manufacturing environments.

The contrast of the display is degraded if there is more than asubstantial monolayer of N*LC domains. The term “substantial monolayer”is defined by the Applicants to mean that, in a direction perpendicularto the plane of the display, there is no more than a single layer ofdomains sandwiched between the electrodes at most points of the display(or the imaging layer), preferably at 75 percent or more of the points(or area) of the display, most preferably at 90 percent or more of thepoints (or area) of the display. In other words, at most, only a minorportion (preferably less than 10 percent) of the points (or area) of thedisplay has more than a single domain (two or more domains) between theelectrodes in a direction perpendicular to the plane of the display,compared to the amount of points (or area) of the display at which thereis only a single domain between the electrodes.

The amount of material needed for a monolayer can be accuratelydetermined by calculation based on individual domain size, assuming afully closed packed arrangement of domains. (In practice, there may beimperfections in which gaps occur and some unevenness due to overlappingdroplets or domains.) On this basis, the calculated amount is preferablyless than about 150 percent of the amount needed for monolayer domaincoverage, preferably not more than about 125 percent of the amountneeded for a monolayer domain coverage, more preferably not more than110 percent of the amount needed for a monolayer of domains.Furthermore, improved viewing angle and broadband features may beobtained by appropriate choice of differently doped domains based on thegeometry of the coated droplet and the Bragg reflection condition.

In a preferred embodiment of the invention, the display device ordisplay sheet has simply a single imaging layer of liquid crystalmaterial along a line perpendicular to the face of the display,preferably a single layer coated on a flexible substrate. Such asstructure, as compared to vertically stacked imaging layers each betweenopposing substrates, is especially advantageous for monochrome shelflabels and the like. Structures having stacked imaging layers, however,are optional for providing additional advantages in some case.

Preferably, the domains are flattened spheres and have on average athickness substantially less than their length, preferably at least 50%less. More preferably, the domains on average have a thickness (depth)to length ratio of 1:2 to 1:6. The flattening of the domains can beachieved by proper formulation and sufficiently rapid drying of thecoating. The domains preferably have an average diameter of 2 to 30microns. The imaging layer preferably has a thickness of 10 to 150microns when first coated and 2 to 20 microns when dried.

The flattened domains of liquid crystal material can be defined ashaving a major axis and a minor axis. In a preferred embodiment of adisplay or display sheet, the major axis is larger in size than the cell(or imaging layer) thickness for a majority of the domains. Such adimensional relationship is shown in U.S. Pat. No. 6,061,107, herebyincorporated by reference in its entirety.

In a typical matrix-addressable light emitting display device, numerouslight emitting devices are formed on a single substrate and arranged ingroups in a regular grid pattern. Activation may be by rows and columns,or in an active matrix with individual cathode and anode paths. OLEDsare often manufactured by first depositing a transparent electrode onthe substrate, and patterning the same into electrode portions. Theorganic layer(s) is then deposited over the transparent electrode. Ametallic electrode can be formed over the electrode layers. For example,in U.S. Pat. No. 5,703,436 to Forrest et al., incorporated herein byreference, transparent indium tin oxide (ITO) is used as the holeinjecting electrode, and a Mg—Ag—ITO electrode layer is used forelectron injection. For purposes of the present invention,matrix-addressable refers to displays which are patterned and typicallyuse rows and columns to produce pixels to form a pattern. However, thepresent use also includes patterns such as icons and characters as wellas segmented displays.

Modern chiral nematic liquid crystal materials usually include at leastone nematic host combined with a chiral dopant. In general, the nematicliquid crystal phase is composed of one or more mesogenic componentscombined to provide useful composite properties. Many such materials areavailable commercially. The nematic component of the chiral nematicliquid crystal mixture may be comprised of any suitable nematic liquidcrystal mixture or composition having appropriate liquid crystalcharacteristics. Nematic liquid crystals suitable for use in the presentinvention are preferably composed of compounds of low molecular weightselected from nematic or nematogenic substances, for example from theknown classes of the azoxybenzenes, benzylideneanilines, biphenyls,terphenyls, phenyl or cyclohexyl benzoates, phenyl or cyclohexyl estersof cyclohexanecarboxylic acid; phenyl or cyclohexyl esters ofcyclohexylbenzoic acid; phenyl or cyclohexyl esters ofcyclohexylcyclohexanecarboxylic acid; cyclohexylphenyl esters of benzoicacid, of cyclohexanecarboxyiic acid and ofcyclohexylcyclohexanecarboxylic acid; phenyl cyclohexanes;cyclohexylbiphenyls; phenyl cyclohexylcyclohexanes;cyclohexylcyclohexanes; cyclohexylcyclohexenes;cyclohexylcyclohexylcyclohexenes; 1,4-bis-cyclohexylbenzenes;4,4-bis-cyclohexylbiphenyls; phenyl- or cyclohexylpyrimidines; phenyl-or cyclohexylpyridines; phenyl- or cyclohexylpyridazines; phenyl- orcyclohexyldioxanes; phenyl- or cyclohexyl-1,3-dithianes;1,2-diphenylethanes; 1,2-dicyclohexylethanes;1-phenyl-2-cyclohexylethanes;1-cyclohexyl-2-(4-phenylcyclohexyl)ethanes;1-cyclohexyl-2′,2-biphenylethanes; 1-phenyl-2-cyclohexylphenylethanes;optionally halogenated stilbenes; benzyl phenyl ethers; tolanes;substituted cinnamic acids and esters; and further classes of nematic ornematogenic substances. The 1,4-phenylene groups in these compounds mayalso be laterally mono- or difluorinated. The liquid crystallinematerial of this preferred embodiment is based on the achiral compoundsof this type. The most important compounds, that are possible ascomponents of these liquid crystalline materials, can be characterizedby the following formula R′—X—Y-Z-R″ wherein X and Z, which may beidentical or different, are in each case, independently from oneanother, a bivalent radical from the group formed by -Phe-, -Cyc-,-Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -Pyr-, -Dio-, -B-Phe- and -B-Cyc-;wherein Phe is unsubstituted or fluorine substituted 1,4-phenylene, Cycis trans-1,4-cyclohexylene or 1,4-cyclohexenylene, Pyr ispyrimidine-2,5-diyl or pyridine-2,5-diyl, Dio is 1,3-dioxane-2,5-diyl,and B is 2-(trans-1,4-cyclohexyl)ethyl, pyrimidine-2,5-diyl,pyridine-2,5-diyl or 1,3-dioxane-2,5-diyl. Y in these compounds isselected from the following bivalent groups —CH═CH—, —C≡C—, —N═N(O)—,—CH═CY′—, —CH═N(O)—, —CH2-CH2-, —CO—O—, —CH2-O—, —CO—S—, —CH2-S—,—COO-Phe-COO— or a single bond, with Y′ being halogen, preferablychlorine, or —CN; R′ and R″ are, in each case, independently of oneanother, alkyl, alkenyl, alkoxy, alkenyloxy, alkanoyloxy, alkoxycarbonylor alkoxycarbonyloxy with 1 to 18, preferably 1 to 12 C atoms, oralternatively one of R′ and R″ is —F, —CF3, —OCF3, —Cl, —NCS or —CN. Inmost of these compounds R′ and R′ are, in each case, independently ofeach another, alkyl, alkenyl or alkoxy with different chain length,wherein the sum of C atoms in nematic media generally is between 2 and9, preferably between 2 and 7. The nematic liquid crystal phasestypically consist of 2 to 20, preferably 2 to 15 components. The abovelist of materials is not intended to be exhaustive or limiting. Thelists disclose a variety of representative materials suitable for use ormixtures, which comprise the active element in electro-optic liquidcrystal compositions.

Suitable chiral nematic liquid crystal compositions preferably have apositive dielectric anisotropy and include chiral material in an amounteffective to form focal conic and twisted planar textures. Chiralnematic liquid crystal materials are preferred because of theirexcellent reflective characteristics, bistability and gray scale memory.The chiral nematic liquid crystal is typically a mixture of nematicliquid crystal and chiral material in an amount sufficient to producethe desired pitch length. Suitable commercial nematic liquid crystalsinclude, for example, E7, E44, E48, E31, E80, BL087, BL101, ZLI-3308,ZLI-3273, ZLI-5048-000, ZLI-5049-100, ZLI-5100-100, ZLI-5800-000,MLC-6041-100.TL202, TL203, TL204 and TL205 manufactured by E. Merck(Darmstadt, Germany). Although nematic liquid crystals having positivedielectric anisotropy, and especially cyanobiphenyls, are preferred,virtually any nematic liquid crystal known in the art, including thosehaving negative dielectric anisotropy should be suitable for use in theinvention. Other nematic materials may also be suitable for use in thepresent invention as would be appreciated by those skilled in the art.

The chiral dopant added to the nematic mixture to induce the helicaltwisting of the mesophase, thereby allowing reflection of visible light,can be of any useful structural class. The choice of dopant depends uponseveral characteristics including among others its chemicalcompatibility with the nematic host, helical twisting power, temperaturesensitivity, and light fastness. Many chiral dopant classes are known inthe art: e.g., G. Gottarelli and G. Spada, Mol. Cryst. Liq. Crys., 123,377 (1985); G. Spada and G. Proni, Enantiomer, 3, 301 (1998) andreferences therein. Typical well known dopant classes include1,1-binaphthol derivatives; isosorbide and similar isomannide esters asdisclosed in U.S. Pat. No. 6,217,792; TADDOL derivatives as disclosed inU.S. Pat. No. 6,099,751; and the pending spiroindanes esters asdisclosed in U.S. patent application Ser. No. 10/651,692 by T. Welter etal., filed Aug. 29, 2003, titled “Chiral Compounds And CompositionsContaining The Same,” hereby incorporated by reference.

The pitch length of the liquid crystal materials may be adjusted basedupon the following equation (1):k _(max) =n ^(av) P ₀where λ_(max) is the peak reflection wavelength, that is, the wavelengthat which reflectance is a maximum, n_(av) is the average index ofrefraction of the liquid crystal material, and p₀ is the natural pitchlength of the chiral nematic helix. Definitions of chiral nematic helixand pitch length and methods of its measurement, are known to thoseskilled in the art such as can be found in the book, Blinov, L. M.,Electro-optical and Magneto-Optical Properties of Liquid Crystals, JohnWiley & Sons Ltd. 1983. The pitch length is modified by adjusting theconcentration of the chiral material in the liquid crystal material. Formost concentrations of chiral dopants, the pitch length induced by thedopant is inversely proportional to the concentration of the dopant. Theproportionality constant is given by the following equation (2):p ₀=1/(HTP.c)where c is the concentration of the chiral dopant and HTP is theproportionality constant.

For some applications, it is desired to have liquid crystal mixturesthat exhibit a strong helical twist and thereby a short pitch length.For example in liquid crystalline mixtures that are used in selectivelyreflecting chiral nematic displays, the pitch has to be selected suchthat the maximum of the wavelength reflected by the chiral nematic helixis in the range of visible light. Other possible applications arepolymer films with a chiral liquid crystalline phase for opticalelements, such as chiral nematic broadband polarizers, filter arrays, orchiral liquid crystalline retardation films. Among these are active andpassive optical elements or color filters and liquid crystal displays,for example STN, TN, AMD-TN, temperature compensation, polymer free orpolymer stabilized chiral nematic texture (PFCT, PSCT) displays.Possible display industry applications include ultralight, flexible, andinexpensive displays for notebook and desktop computers, instrumentpanels, video game machines, videophones, mobile phones, hand held PCs,PDAs, e-books, camcorders, satellite navigation systems, store andsupermarket pricing systems, highway signs, informational displays,smart cards, toys, and other electronic devices.

Chiral nematic liquid crystal materials and cells, as well as polymerstabilized chiral nematic liquid crystals and cells, are well known inthe art and described in, for example, U.S. application Ser. No.07/969,093 and Ser. No. 08/057,662; Yang et al., Appl. Phys. Lett.60(25) pp 3102-04 (1992); Yang et al., J. Appl. Phys. 76(2) pp 1331(1994); published International Patent Application No. PCT/US92/09367;and published International Patent Application No. PCT/US92/03504, allof which are incorporated herein by reference.

In a preferred embodiment, a light modulating layer is deposited over afirst conductor. The light modulating layer contains a chiral nematicliquid crystal. The selected material preferably exhibits high opticaland electrical anisotropy and matches the index of refraction of thecarrier polymer, when the material is electrically oriented. Examples ofsuch materials are E. Merck's BL-03, BL-048 or BL-033, which areavailable from EM Industries of Hawthorne, N.Y. Other light reflectingor diffusing modulating, electrically operated materials can also becoated, such as a micro-encapsulated electrophoretic material in oil.

The liquid crystal can be a chiral doped nematic liquid crystal, alsoknown as cholesteric liquid crystal, such as those disclosed in U.S.Pat. No. 5,695,682, incorporated herein by reference. Application offields of various intensity and duration change the state of chiraldoped nematic materials from a reflective to a transmissive state. Thesematerials have the advantage of maintaining a given state indefinitelyafter the field is removed. Cholesteric liquid crystal materials can beMerck BL112, BL118 or BL126 that are available from EM Industries ofHawthorne, N.Y. The light modulating layer is effective in twoconditions.

Liquid crystal domains may be preferably made using a limitedcoalescence methodology, as disclosed in U.S. Pat. Nos. 6,556,262 and6,423,368, incorporated herein by reference. Limited coalescence isdefined as dispersing a light modulating material below a given size,and using coalescent limiting material to limit the size of theresulting domains. Such materials are characterized as having a ratio ofmaximum to minimum domain size of less than 2:1. By use of the term“uniform domains”, it is meant that domains are formed having a domainsize variation of less than 2:1. Limited domain materials have improvedoptical properties.

An immiscible, field responsive light modulating material along with aquantity of colloidal particles is dispersed in an aqueous system andblended to form a dispersion of field responsive, light modulatingmaterial below a coalescence size. When the emulsion, also referred toherein as a dispersion, is dried, a coated material is produced whichhas a set of uniform domains having a plurality of electricallyresponsive optical states. The colloidal solid particle, functioning asan emulsifier, limits domain growth from a highly dispersed state.Uniformly sized liquid crystal domains are created and machine coated tomanufacture light modulating, electrically responsive sheets withimproved optical efficiency.

Specifically, a liquid crystal material may be dispersed an aqueous bathcontaining a water soluble binder material such as deionized gelatin,polyvinyl alcohol (PVA) or polyethylene oxide (PEO). Such compounds aremachine coatable on equipment associated with photographic films.Preferably, the binder has a low ionic content, as the presence of ionsin such a binder hinders the development of an electrical field acrossthe dispersed liquid crystal material. Additionally, ions in the bindercan migrate in the presence of an electrical field, chemically damagingthe light modulating layer. The liquid crystal/gelatin emulsion iscoated to a thickness of between 5 and 30 microns to optimize opticalproperties of light modulating layer. The coating thickness, size of theliquid crystal domains, and concentration of the domains of liquidcrystal materials are designed for optimum optical properties.

In an exemplary embodiment, a liquid crystalline material is homogenizedin the presence of finely divided silica, a coalescence limitingmaterial, (LUDOX® from duPont Corporation). A promoter material, such asa copolymer of adipic acid and 2-(methylamino) ethanol, is added to theaqueous bath to drive the colloidal particles to the liquid-liquidinterface. The liquid crystal material is dispersed using ultrasound tocreate liquid crystal domains below 1 micron in size. When theultrasound energy was removed, the liquid crystal material coalescedinto domains of uniform size. The ratio of smallest to largest domainsize varied by approximately 1:2. By varying the amount of silica andcopolymer relative to the liquid crystalline material, uniform domainsize emulsions of average diameter (by microscopy) approximately 1, 3,and, 8 micron were produced. These emulsions were diluted into gelatinsolution for subsequent coating.

Domains of a limited coalescent material maintain their uniform sizeafter the addition of the surfactant and after being machine coated.There were few, if any, parasitic domains having undesirableelectro-optical properties within the dried coatings produced by thelimited coalescence method. Coatings made using limited coalescencehaving a domain size of about 2 microns may have the greatesttranslucence. For constant material concentrations and coatingthickness, limited coalescent materials having a domain size of about 2microns in size are significantly more translucent than any sizeddomains formed using conventional dispersion.

Sheets made by the limited coalescence process have curves similar tothose of conventionally dispersed materials. However, with 8 to 10micron domains, the material may demonstrate reduced scattering due tothe elimination of parasitic domains. Conventionally dispersedcholesteric materials contain parasitic domains, which reflect light inwavelengths outside the wavelengths reflected by the cholestericmaterial. Limited coalescent dispersions have reduced reflection inother wavelengths due to the elimination of parasitic domains. Theincreased purity of color is important in the development of full colordisplays requiring well separated color channels to create a full colorimage. Limited coalescent cholesteric materials provide purer lightreflectance than cholesteric liquid crystal material dispersed byconventional methods. Such materials may be produced using conventionalphotographic coating machinery.

In order to provide suitable formulations for applying a layercontaining the liquid crystal domains, the dispersions are combined witha hydrophilic colloid, gelatin being the preferred material. Surfactantsmay be included with the liquid crystal dispersion prior to the additionof gelatin in order to prevent the removal of the particulate suspensionstabilizing agent from the droplets. This aids in preventing furthercoalescence of the droplets.

As for the suspension stabilizing agents that surround and serve toprevent the coalescence of the droplets, any suitable colloidalstabilizing agent known in the art of forming polymeric particles by theaddition reaction of ethylenically unsaturated monomers by the limitedcoalescence technique can be employed, such as, for example, inorganicmaterials such as, metal salt or hydroxides or oxides or clays, organicmaterials such as starches, sulfonated crosslinked organic homopolymersand resinous polymers as described, for example, in U.S. Pat. No.2,932,629; silica as described in U.S. Pat. No. 4,833,060; copolymerssuch as copoly(styrene-2-hydroxyethyl methacrylate-methacrylicacid-ethylene glycol dimethacrylate) as described in U.S. Pat. No.4,965,131, all of which are incorporated herein by reference. Silica isthe preferred suspension stabilizing agent.

Suitable hydrophilic binders include both naturally occurring substancessuch as proteins, protein derivatives, cellulose derivatives (e.g.cellulose esters), gelatins and gelatin derivatives, polysaccaharides,casein, and the like, and synthetic water permeable colloids such aspoly(vinyl lactams), acrylamide polymers, poly(vinyl alcohol) and itsderivatives, hydrolyzed polyvinyl acetates, polymers of alkyl andsulfoalkyl acrylates and methacrylates, polyamides, polyvinyl pyridine,acrylic acid polymers, maleic anhydride copolymers, polyalkylene oxide,methacrylamide copolymers, polyvinyl oxazolidinones, maleic acidcopolymers, vinyl amine copolymers, methacrylic acid copolymers,acryloyloxyalkyl acrylate and methacrylates, vinyl imidazole copolymers,vinyl sulfide copolymers, and homopolymer or copolymers containingstyrene sulfonic acid. Gelatin is preferred.

The flexible plastic substrate can be any flexible self supportingplastic film that supports the thin conductive metallic film. “Plastic”means a high polymer, usually made from polymeric synthetic resins,which may be combined with other ingredients, such as curatives,fillers, reinforcing agents, colorants, and plasticizers. Plasticincludes thermoplastic materials and thermosetting materials.

The flexible plastic film must have sufficient thickness and mechanicalintegrity so as to be self supporting, yet should not be so thick as tobe rigid. Typically, the flexible plastic substrate is the thickestlayer of the composite film in thickness. Consequently, the substratedetermines to a large extent the mechanical and thermal stability of thefully structured composite film.

Another significant characteristic of the flexible plastic substratematerial is its glass transition temperature (Tg). Tg is defined as theglass transition temperature at which plastic material will change fromthe glassy state to the rubbery state. It may comprise a range beforethe material may actually flow. Suitable materials for the flexibleplastic substrate include thermoplastics of a relatively low glasstransition temperature, for example up to 150° C., as well as materialsof a higher glass transition temperature, for example, above 150° C. Thechoice of material for the flexible plastic substrate would depend onfactors such as manufacturing process conditions, such as depositiontemperature, and annealing temperature, as well as post-manufacturingconditions such as in a process line of a displays manufacturer. Certainof the plastic substrates discussed below can withstand higherprocessing temperatures of up to at least about 200° C., some up to300-350° C., without damage.

Typically, the flexible plastic substrate is polyethylene terephthalate(PET), polyethylene naphthalate (PEN), polyethersulfone (PES),polycarbonate (PC), polysulfone, a phenolic resin, an epoxy resin,polyester, polyimide, polyetherester, polyetheramide, cellulose acetate,aliphatic polyurethanes, polyacrylonitrile, polytetrafluoroethylenes,polyvinylidene fluorides, poly(methyl x-methacrylates), an aliphatic orcyclic polyolefin, polyarylate (PAR), polyetherimide (PEI),polyethersulphone (PES), polyimide (PI), Teflon poly(perfluoro-alboxy)fluoropolymer (PFA), poly(ether ether ketone) (PEEK), poly(ether ketone)(PEK), poly(ethylene tetrafluoroethylene)fluoropolymer (PETFE), andpoly(methyl methacrylate) and various acrylate/methacrylate copolymers(PMMA). Aliphatic polyolefins may include high density polyethylene(HDPE), low density polyethylene (LDPE), and polypropylene, includingoriented polypropylene (OPP). Cyclic polyolefins may includepoly(bis(cyclopentadiene)). A preferred flexible plastic substrate is acyclic polyolefin or a polyester. Various cyclic polyolefins aresuitable for the flexible plastic substrate. Examples include Arton®made by Japan Synthetic Rubber Co., Tokyo, Japan; Zeanor T made by ZeonChemicals L.P., Tokyo Japan; and Topas® made by Celanese A. G., KronbergGermany. Arton is a poly(bis(cyclopentadiene)) condensate that is a filmof a polymer. Alternatively, the flexible plastic substrate can be apolyester. A preferred polyester is an aromatic polyester such asArylite. Although various examples of plastic substrates are set forthabove, it should be appreciated that the substrate can also be formedfrom other materials such as glass and quartz.

The flexible plastic substrate can be reinforced with a hard coating.Typically, the hard coating is an acrylic coating. Such a hard coatingtypically has a thickness of from 1 to 15 microns, preferably from 2 to4 microns and can be provided by free radical polymerization, initiatedeither thermally or by ultraviolet radiation, of an appropriatepolymerizable material. Depending on the substrate, different hardcoatings can be used. When the substrate is polyester or Arton, aparticularly preferred hard coating is the coating known as “Lintec”.Lintec contains UV cured polyester acrylate and colloidal silica. Whendeposited on Arton, it has a surface composition of 35 atom % C, 45 atom% 0, and 20 atom % Si, excluding hydrogen. Another particularlypreferred hard coating is the acrylic coating sold under the trademark“Terrapin” by Tekra Corporation, New Berlin, Wis.

In one embodiment, a sheet supports a conventional polymer dispersedlight modulating material. The sheet includes a substrate. The substratemay be made of a polymeric material, such as Kodak Estar film baseformed of polyester plastic, and have a thickness of between 20 and 200microns. For example, the substrate may be an 80 micron thick sheet oftransparent polyester. Other polymers, such as transparentpolycarbonate, can also be used. Alternatively, the substrate 15 may bethin, transparent glass.

The liquid crystal display contains at least one conductive layer, whichtypically is comprised of a primary metal oxide. This conductive layermay comprise other metal oxides such as indium oxide, titanium dioxide,cadmium oxide, gallium indium oxide, niobium pentoxide and tin dioxide.See, Int. Publ. No. WO 99/36261 by Polaroid Corporation, incorporatedherein by reference. In addition to the primary oxide such as ITO, theat least one conductive layer can also comprise a secondary metal oxidesuch as an oxide of cerium, titanium, zirconium, hafnium and/ortantalum. See, U.S. Pat. No. 5,667,853 to Fukuyoshi et al. (ToppanPrinting Co.), incorporated herein by reference. Other transparentconductive oxides include, but are not limited to ZnO₂, Zn₂SnO₄,Cd₂SnO₄, Zn₂In₂O₅, MgIn₂O₄, Ga₂O₃—In₂O₃, or TaO₃. The conductive layermay be formed, for example, by a low temperature sputtering technique orby a direct current sputtering technique, such as DC-sputtering or RF-DCsputtering, depending upon the material or materials of the underlyinglayer. The conductive layer may be a transparent, electricallyconductive layer of tin oxide or indium-tin oxide (ITO), orpolythiophene, with ITO being the preferred material. Typically, theconductive layer is sputtered onto the substrate to a resistance of lessthan 300 Ohms per square. Alternatively, conductive layer may be anopaque electrical conductor formed of metal such as copper, aluminum ornickel. If the conductive layer is an opaque metal, the metal can be ametal oxide to create a light absorbing conductive layer.

Indium tin oxide (ITO) is the preferred conductive material, as it is acost effective conductor with good environmental stability, up to 90%transmission, and down to 20 Ohms per square resistivity. An exemplarypreferred ITO layer has a % T greater than or equal to 80% in thevisible region of light, that is, from greater than 400 nm to 700 nm, sothat the film will be useful for display applications. In a preferredembodiment, the conductive layer comprises a layer of low temperatureITO which is polycrystalline. The ITO layer is preferably 10-120 nm inthickness, or 50-100 nm thick to achieve a resistivity of 20-60 Ohms persquare on plastic. An exemplary preferred ITO layer is 60-80 nm thick.

The conductive layer is preferably patterned. The conductive layer ispreferably patterned into a plurality of electrodes. The patternedelectrodes may be used to form a liquid crystal display device. Inanother embodiment, two conductive substrates are positioned facing eachother and cholesteric liquid crystals are positioned therebetween toform a device. The patterned ITO conductive layer may have a variety ofdimensions. Exemplary dimensions may include line widths of 10 microns,distances between lines, that is, electrode widths, of 200 microns,depth of cut, that is, thickness of ITO conductor, of 100 nanometers.ITO thicknesses on the order of 60, 70, and greater than 100 nanometersare also possible.

The display may also contain a second conductive layer applied to thesurface of the light modulating layer. The second conductive layerdesirably has sufficient conductivity to carry a field across the lightmodulating layer. The second conductive layer may be formed in a vacuumenvironment using materials such as aluminum, tin, silver, platinum,carbon, tungsten, molybdenum, or indium. Oxides of these metals can beused to darken patternable conductive layers. The metal material can beexcited by energy from resistance heating, cathodic arc, electron beam,sputtering or magnetron excitation. The second conductive layer maycomprise coatings of tin oxide or indium-tin oxide, resulting in thelayer being transparent. Alternatively, second conductive layer may beprinted conductive ink.

For higher conductivities, the second conductive layer may comprise asilver based layer which contains silver only or silver containing adifferent element such as aluminum (Al), copper (Cu), nickel (Ni),cadmium (Cd), gold (Au), zinc (Zn), magnesium (Mg), tin (Sn), indium(In), tantalum (Ta), titanium (Ti), zirconium (Zr), cerium (Ce), silicon(Si), lead (Pb) or palladium (Pd). In a preferred embodiment, theconductive layer comprises at least one of gold, silver and agold/silver alloy, for example, a layer of silver coated on one or bothsides with a thinner layer of gold. See, Int. Publ. No. WO 99/36261 byPolaroid Corporation, incorporated herein by reference. In anotherembodiment, the conductive layer may comprise a layer of silver alloy,for example, a layer of silver coated on one or both sides with a layerof indium cerium oxide (InCeO). See U.S. Pat. No. 5,667,853,incorporated herein in by reference.

The second conductive layer may be patterned irradiating themultilayered conductor/substrate structure with ultraviolet radiation sothat portions of the conductive layer are ablated therefrom. It is alsoknown to employ an infrared (IR) fiber laser for patterning a metallicconductive layer overlying a plastic film, directly ablating theconductive layer by scanning a pattern over the conductor/filmstructure. See: Int. Publ. No. WO 99/36261 and “42.2: A New ConductorStructure for Plastic LCD Applications Utilizing ‘All Dry’ Digital LaserPatterning,” 1998 SID International Symposium Digest of TechnicalPapers, Anaheim, Calif., May 17-22, 1998, no. VOL. 29, May 17, 1998,pages 1099-1101, both incorporated herein by reference.

In addition to a second conductive layer, other means may be used toproduce a field capable of switching the state of the liquid crystallayer as described in, for example, U.S. Pat Appl. Nos. 20010008582 A1,20030227441 A1, 20010006389 A1, and U.S. Pat. Nos. 6,424,387, 6,269,225,and 6,104,448, all incorporated herein by reference.

The liquid crystal display may also comprises at least one “functionallayer” between the conductive layer and the substrate. The functionallayer may comprise a protective layer or a barrier layer. The protectivelayer useful in the practice of the invention can be applied in any of anumber of well known techniques, such as dip coating, rod coating, bladecoating, air knife coating, gravure coating and reverse roll coating,extrusion coating, slide coating, curtain coating, and the like. Theliquid crystal particles and the binder are preferably mixed together ina liquid medium to form a coating composition. The liquid medium may bea medium such as water or other aqueous solutions in which thehydrophilic colloid are dispersed with or without the presence ofsurfactants. A preferred barrier layer may acts as a gas barrier or amoisture barrier and may comprise SiOx, AlOx or ITO. The protectivelayer, for example, an acrylic hard coat, functions to prevent laserlight from penetrating to functional layers between the protective layerand the substrate, thereby protecting both the barrier layer and thesubstrate. The functional layer may also serve as an adhesion promoterof the conductive layer to the substrate.

In another embodiment, the polymeric support may further comprise anantistatic layer to manage unwanted charge build up on the sheet or webduring roll conveyance or sheet finishing. In another embodiment of thisinvention, the antistatic layer has a surface resistivity of between 10⁵to 10¹². Above 10¹², the antistatic layer typically does not providesufficient conduction of charge to prevent charge accumulation to thepoint of preventing fog in photographic systems or from unwanted pointswitching in liquid crystal displays. While layers greater than 10⁵ willprevent charge buildup, most antistatic materials are inherently notthat conductive and in those materials that are more conductive than10⁵, there is usually some color associated with them that will reducethe overall transmission properties of the display. The antistatic layeris separate from the highly conductive layer of ITO and provides thebest static control when it is on the opposite side of the web substratefrom that of the ITO layer. This may include the web substrate itself.

Another type of functional layer may be a color contrast layer. Colorcontrast layers may be radiation reflective layers or radiationabsorbing layers. In some cases, the rearmost substrate of each displaymay preferably be painted black. The color contrast layer may also beother colors. In another embodiment, the dark layer comprises millednon-conductive pigments. The materials are milled below 1 micron to form“nano-pigments”. In a preferred embodiment, the dark layer absorbs allwavelengths of light across the visible light spectrum, that is from 400nanometers to 700 nanometers wavelength. The dark layer may also containa set or multiple pigment dispersions. Suitable pigments used in thecolor contrast layer may be any colored materials, which are practicallyinsoluble in the medium in which they are incorporated. Suitablepigments include those described in Industrial Organic Pigments:Production, Properties, Applications by W. Herbst and K. Hunger, 1993,Wiley Publishers. These include, but are not limited to, Azo Pigmentssuch as monoazo yellow and orange, diazo, naphthol, naphthol reds, azolakes, benzimidazolone, diazo condensation, metal complex, isoindolinoneand isoindolinic, polycyclic pigments such as phthalocyanine,quinacridone, perylene, perinone, diketopyrrolo-pyrrole, and thioindigo,and anthriquinone pigments such as anthrapyrimidine.

The functional layer may also comprise a dielectric material. Adielectric layer, for purposes of the present invention, is a layer thatis not conductive or blocks the flow of electricity. This dielectricmaterial may include a UV curable, thermoplastic, screen printablematerial, such as Electrodag 25208 dielectric coating from AchesonCorporation. The dielectric material forms a dielectric layer. Thislayer may include openings to define image areas, which are coincidentwith the openings. Since the image is viewed through a transparentsubstrate, the indicia are mirror imaged. The dielectric material mayform an adhesive layer to subsequently bond a second electrode to thelight modulating layer.

The liquid crystal (LC) is used as an optical switch. The substrates areusually manufactured with transparent, conductive electrodes, in whichelectrical “driving” signals are coupled. The driving signals induce anelectric field which can cause a phase change or state change in theliquid crystal material, the liquid crystal exhibiting different lightreflecting characteristics according to its phase and/or state. Passivematrix displays, having one or more field-spreading layers, whileimproving contrast over prior art elements, do not achieve the bestcontrast unless the correct sequence of drive signals is applied.

The preferred drive method for the invention involves a 4-phaseapproach. In the first phase, the area of the display to be updated isreset to a planar texture. Referring to FIG. 2, an AC pixel voltage isapplied across the pixels such that the critical voltage is reached ifnot exceeded. The duration of the AC pixel voltage is held for a periodsuitable to achieve the homeotropic texture. In phase 2, the pixelvoltage of the display is set to substantially low voltage to allow thehomeotropic domains of the liquid crystal material to relax to thestable planar texture. Phase 3 is the scanning phase, where each row ofthe display to be updated is addressed, preferably sequentiallyaddressed. When the row is addressed, it is said to be “selected,” whileany other row is said to be non-selected. In the selected row, pixelsthat are to be switched from the stable planar texture to thenon-reflective focal conic texture receive a pixel voltage pulse acrossthem greater than V1 to produce the planar to focal conic (P-FC)transition. Pixels that are to remain in the stable planar texturereceive a pulse or set of pulses such that there is negligible effect onthe final texture of the pixel, which is stable planar. After the pixelvoltage pulse or pulses have sufficiently caused the planar -focal conictransition to select pixels in the selected row, the next row to beaddressed is selected. The selection process is repeated until all rowshave been addressed. This drive method or scheme can be described as aplanar reset, left-slope selection method. Finally, all pixel voltagesare removed from the updated area of the display.

Specifically, FIG. 2 represents the stabilized reflectance of chiralnematic liquid crystal after the applied voltage has been removed andthe chiral nematic liquid crystal is allowed to obtain a stable texture.This graph is typically obtained by first applying an AC pixel voltagefor a fixed period of time to reset the display to a known texture,either focal conic or homeotropic. Following the reset period is aperiod where the display is allowed to stabilize into the initialtexture. After the display has stabilized, an AC test voltage is appliedto the chiral nematic liquid crystal for a fixed period of time and thenremoved. After a brief period of relaxation/stablization time, thereflectance of the chiral nematic liquid crystal is measured. A reset tothe initial condition must be performed for every test voltage on thex-axis.

The following examples are provided to illustrate the invention.

EXAMPLE DISPLAY 1

A 30 pixel per inch passive matrix display was created as follows. Fiveinch wide polyethylene terephthalate support, having 300 Ohm per squareITO, (Bekaert Specialty Films) was patterned across the web with afocused laser beam to produce 30 pixel per inch columns separated byabout 100 micron gaps.

A dispersion of the chiral nematic composition was prepared as follows.To 248 grams of distilled water was added 3.7 grams of Ludox® colloidalsilica suspension and 7.6 grams of a 10% w/w aqueous solution of acopolymer of methylaminoethanol and adipic acid. To this was added 111grams of the appropriate amount of liquid crystal BL118 obtained fromMerck, Darmstadt, Germany. The mixture was stirred using aSilverson®mixer at 5000 rpm. It was then passed twice through aMicrofluidizer® at 3000 psi. Three hundred and seventy grams of theresulting dispersion was mixed with 962 grams of an aqueous solutioncontaining 6.8% w/w alkali treated gelatin at 50° C. The dispersion (8%w/w liquid crystal material and 5% w/w gelatin) was stored in arefrigerator until further use. Microscopic examination showed that thedispersion consisted of uniform 8 μm droplets of the liquid crystalmaterial in an aqueous gelatin medium.

The dispersion was warmed back to 40 degrees C. then knife coated ontothe laser etched support to give a coverage of 92.2 cm³/m² (8.57cm³/ft²) for the liquid crystal material. The coatings were allowed todry.

To form a color contrast field-spreading layer, the following materialswere mixed together and applied over the dried liquid crystal layer. ABaytron P HC V2 (obtained from H.C. Starck Inc, Newton, Mass.,) solutionin water was pH adjusted using Triethylamine (CAS 121-44-8) fromapproximately 2.0 to a pH range of 5 to 7 and heated to 40 degreesCelsius. Photographic gelatin was added to deionized water to make a 2%solution by weight and heated to 40 degrees Celsius. The solution wasthen placed into a 40 degree Celsius temperature bath and to this thegelatin and water mixture was slowly added. Once the mixture washomogenized a combination of magenta and cyan non-conductive pigmentsmilled to less than 1 micron in size was added to the solution. To aidin the coating process, the total mixture was diluted to 1.75% gel. TheBaytron P HC V2 constituted about 4% of the final dry coverage of thislayer after coating and drying.

CONTROL DISPLAY 1

A control passive matrix display was prepared exactly as described inexample 1 except that the Baytron P HC V2 was left out of thecontrasting color mixture.

Imaging the Displays

The control and example displays were written using drive methods asenumerated in Table 1. Voltage levels and timing sequences were adjustedto produce a good image on the control display for each drive method.With the control display, the liquid crystal in the gaps, as coated, wasprimarily in the planar state before imaging and remained in the planarstate after imaging regardless of the drive method used. The samesignals were then applied to the display having the field-spreadinglayer. In all cases good contrast was achieved within the pixels of thedisplays. Table 1 and FIG. 3 summarize the results for the gap statesbetween planar pixels (light state neighbors) and between focal conicpixels (dark state neighbors), with the displays having afield-spreading layer. FIG. 3 shows a portion of the imaged displayafter the display was driven with various drive methods. FIG. 3 aillustrates the desired gap reflectivity states, where the state betweenneighboring light pixels is light, and the state between neighboringdark pixels is dark. FIGS. 3 b, 3 c and 3 d illustrate the undesirablegap reflective states. In FIG. 3 b, the gap state is reflectiveregardless of the pixel state. FIG. 3 c shows the dark gaps everywhere,regardless of the reflectivity of the pixels. FIG. 3 d illustrates theopposite of the desired gap states, where the gaps are light betweendark pixels, and dark between light pixels.

Drive method 1 in Table 1 follows a conventional drive method. Theentire display panel was first reset into the focal conic texture byapplying a 90V AC 250 Hz square wave between rows and columns for aperiod of 100 milliseconds (ms). Following the reset was a relaxationperiod of zero volts across the entire display panel for 200 ms. Thenext phase involves updating the display line by line by addressing eachrow as a selected row. All other rows are considered non-selected. Theselection method utilizes the right slope of the electro-opticalresponse curve shown in FIG. 2. The pixels in the selected row receivean AC voltage between the row and columns for period of 40 ms at afrequency of 250 Hz. 140 V was applied to the pixels in the selected rowthat were to obtain the final stable planar texture. For pixels in theselected row to remain in the focal conic texture, 90 V was applied.Pixels in the non-selected rows experienced an AC voltage of 25 V at afrequency of 250 Hz for the duration of the updating process. After allrows have been addressed, all voltages were removed from the displaypanel, allowing all pixels to stabilize into their final reflectivestates. The display image and resulting gap states are shown in FIG. 3c.

Drive method 2 in Table 1 follows a conventional drive method as well.The entire display panel was first reset into the planar texture byapplying a 150 V AC 250 Hz square wave between rows and columns for aperiod of 100 ms. Following the reset was a relaxation period of zerovolts across the entire display panel for 200 ms. The next phaseinvolves updating the display line by line by addressing each row as aselected row. All other rows are considered non-selected. The selectionmethod utilizes the right slope of the electro-optical response curveshown in FIG. 2. The pixels in the selected row receive an AC voltagebetween the row and columns for period of 40 ms at a frequency of 250Hz. 130 V was applied to the pixels in the selected row that were toobtain the final stable planar texture. For pixels in the selected rowto remain in the focal conic texture, 80 V was applied. Pixels in thenon-selected rows experienced an AC voltage of 25 V at a frequency of250 Hz for the duration of the updating process. After all rows havebeen addressed, all voltages were removed from the display panel,allowing all pixels to stabilize into their final reflective states. Thedisplay image and resulting gap states are shown in FIG. 3 d.

Drive method 3 in Table 1 follows a conventional pulse accumulationdrive method. This method involves updating the display line by line byaddressing each row as a selected row. All other rows are considerednon-selected. The selection method utilizes the right slope of theelectro-optical response curve shown in FIG. 2. The pixels in theselected row received a single AC voltage pulse cycle between the rowand columns for period of 5 ms. 170 V was applied to the pixels in theselected row that were to obtain the final stable planar texture. Forpixels in the selected row to switch to or remain in the focal conictexture, 120 V was applied. Pixels in the non-selected rows experiencedan AC voltage of 25 V at a frequency of 200 Hz for the duration of theupdating process. After all rows had been addressed, the display updateprocess was repeated 5 more times, allowing pixels in the display tocumulatively transition from their initial texture to their finaltexture. After the 6^(th) pass of display updates, all voltages wereremoved from the display panel, allowing all pixels to stabilize intotheir final reflective states. The display image and resulting gapstates are shown in FIG. 3 c.

Dynamic drive method 4 in Table 1 utilizes the referenced U/√2 drivemethod. This drive method utilizes only 2 drive voltages, U and 0, forboth row and column. In this implementation, U was 120 V. In the firstphase of the drive method, the entire display panel was first preparedinto the homeotropic texture by applying the AC voltage U at 250 Hz fora period of 100 ms. The next phase of the drive method is theholding/selection phase. On a row-by-row basis, each row was addressedas the selected row, while all other rows were considered non-selected.Pixels in the selected row received a voltage of either U or 0 V at 100%duty cycle, which causes the chiral nematic liquid crystal domains toremain in the homeotropic texture or transition into the transientplanar texture, respectively. The pixels to obtain the final stableplanar texture received the voltage U, while pixel to transition intofocal conic received the voltage of 0 V. The duration of the selectiontime was 1 ms. All pixels in the non-selected rows received a voltage ofU at 50% duty cycle, which produces a local root mean square (RMS)voltage of U/√2. This voltage has the effect of either holding thechiral nematic liquid crystal domains in the homeotropic texture if theywere previously in the homeotropic texture, or evolving them into thefocal conic texture if they were previously in the transient planartexture. In order for the last few rows to transition correctly, anevolution phase was applied after all rows had been addressed. Thisconsisted of applying a voltage of U at 50% duty cycle across the entiredisplay panel for 15 ms. After the last evolution phase was performed,all voltages were removed from the display panel, allowing all pixels tostabilize into their final reflective states. The display image andresulting gap states are shown in FIG. 3 b.

Drive method 5 in Table 1, the preferred method, follows a conventionaldrive method. The entire display panel was first reset into the planartexture by applying a 140 V AC 250 Hz square wave between rows andcolumns for a period of 100 ms. Following the reset was a relaxationperiod of zero volts across the entire display panel for 200 ms. Thenext phase involves updating the display line by line by addressing eachrow as a selected row. All other rows are considered non-selected. Theselection method utilizes the left slope of the electro-optical responsecurve shown in FIG. 2. The pixels in the selected row receive an ACvoltage between the row and columns for period of 40 ms at a frequencyof 250 Hz. 25 V was applied to the pixels in the selected row that wereto remain in the stable planar texture. For pixels in the selected rowto transition to the focal conic texture, 75 V was applied. Pixels inthe non-selected rows experienced an AC voltage of 25 V at a frequencyof 250 Hz for the duration of the updating process. After all rows havebeen addressed, all voltages were removed from the display panel,allowing all pixels to stabilize into their final reflective states. Thedisplay image and resulting gap states are shown in FIG. 3 a.

Drive method 5 can have many variations. For example, the time totransition the pixels from stable planar to focal conic can be reducedby applying a selection voltage that is greater than V2 of FIG. 2. Thevoltage following this shortened high voltage pulse can be zero volts orit can be some holding voltage, as described in U.S. Patent application2005/0024307 A1, incorporated herein by reference. There can be caseswhere there are multiple pulses. It is understood that in all caseswhere the display is first reset into the stable planar texture and thenupdate by means of transitioning select pixels to the focal conictexture is the enabling feature of this invention. TABLE 1 Gap states ofa passive matrix display having a field-spreading layer written withselected drive methods. Dark Light # Drive Method Scope Gaps¹ Gaps²Desirability 1 Conventional, Check Dark Dark Low FC Reset, Right- SlopeSelect 2 Conventional, Check Light Dark Very Low Planar Reset,Right-Slope Select 3 Pulse Accumu- Check Dark Dark Low lation, Right-Slope Select 4 U/√2 Dynamic Check Light Light Low 5 Planar Reset,Invention Dark Light Very High Left-Slope Select¹State achieved in row gaps between neighboring dark pixels.²State achieved in row gaps between neighboring light pixels.Although all these drive methods successfully imaged the passive matrixcholesteric displays with or without field-spreading layers, theunexpected result is that only methods having planar reset and aleft-slope selection phases achieve the most desired combination oflight and dark gaps necessary for optimum contrast when uses incombination with displays having at least one field-spreading layer. Themost desired pixel and gap combination is one of having a light gapbetween neighboring light pixel and a dark gap between neighboring darkpixels.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

1. A bistable matrix-addressable display element comprising a substrate,a bistable electrically modulated imaging layer, at least one conductor,and at least one field-spreading layer between said bistableelectrically modulated imaging layer and said at least one conductor,wherein said field-spreading layer has a sheet resistance (SER) of from10⁹ to 10⁶ Ohms per square.
 2. The bistable matrix-addressable displayelement of claim 1 wherein said bistable electrically modulated imaginglayer comprises a liquid crystal material.
 3. The bistablematrix-addressable display element of claim 1 wherein said bistableelectrically modulated imaging layer comprises a polymer dispersedchiral nematic liquid crystal material.
 4. The bistablematrix-addressable display element of claim 1 wherein said bistableelectrically modulated imaging layer comprises an electrophoreticmaterial.
 5. The bistable matrix-addressable display element of claim 1wherein said bistable electrically modulated imaging layer comprises afield or voltage driven switching layer.
 6. The bistablematrix-addressable display element of claim 1 wherein saidfield-spreading layer comprises a colorant having a contrasting color tothe reflection maximum of said bistable electrically modulated imaginglayer.
 7. The bistable matrix-addressable display element of claim 1wherein said a field-spreading layer is transparent.
 8. The bistablematrix-addressable display element of claim 1 wherein saidfield-spreading layer comprises at least one member selected from thegroup consisting of conjugated conducting polymers, conducting carbonparticles, crystalline semiconductor particles, amorphous semiconductivefibrils, continuous conductive metal and semiconducting thin films. 9.The bistable matrix-addressable display element of claim 1 wherein saidfield-spreading layer comprises electronically conductivemetal-containing particles.
 10. The bistable matrix-addressable displayelement of claim 9 wherein said electronically conductivemetal-containing particles are semiconducting metal oxides.
 11. Thebistable matrix-addressable display element of claim 9 wherein saidelectronically conductive metal-containing particles are conductivecrystalline inorganic oxides, conductive metal antimonates, orconductive inorganic non-oxides.
 12. The bistable matrix-addressabledisplay element of claim 1 wherein said field-spreading layer compriseselectronically conductive polymer.
 13. The bistable matrix-addressabledisplay element of claim 12 wherein said and electronically conductivepolymer is at least one member selected from the group consisting ofsubstituted or unsubstituted polythiophenes, substituted orunsubstituted polypyrroles, and substituted or unsubstitutedpolyanilines.
 14. The bistable matrix-addressable display element ofclaim 1 wherein said at least one conductor is a first conductor locatedbetween said substrate and said bistable electrically modulated imaginglayer.
 15. The bistable matrix-addressable display element of claim 14further comprising at least a second conductor on the side of saidbistable electrically modulated imaging layer opposite said firstconductor.
 16. The bistable matrix-addressable display element of claim1 wherein said at least one conductor is a conductor located on the sideof said bistable electrically modulated imaging layer opposite saidsubstrate.
 17. A method of imaging a bistable matrix-addressable displayelement comprising: providing a bistable matrix-addressable displayelement comprising a substrate, a bistable chiral nematic liquid crystalimaging layer having a reflection maximum, at least one conductor, andat least one field-spreading layer between said bistable chiral nematicliquid crystal imaging layer and said at least one conductor, whereinsaid field-spreading layer has a sheet resistance (SER) of from 10⁹ to10⁶ Ohms per square; identifying an area to be updated of said bistablematrix-addressable display element, wherein said area to be updatedcomprises rows of pixels; and applying a sequence of drive signalshaving a 4-phase approach to image said bistable matrix-addressabledisplay element, wherein said 4-phase approach comprises: in phase 1,applying a first pixel voltage across said pixels of said area to beupdated such that the critical voltage is reached; and holding saidfirst pixel voltage until a homeotropic texture is reached; in phase 2,setting a second pixel voltage to allow said homeotropic texture torelax into a stable planar texture; in phase 3, selecting one row ofpixels of said rows of pixels of said area to be updated; and updatingsaid one row of pixels by addressing, wherein addressing comprisesapplying a third pixel voltage, capable of switching said pixels fromsaid stable planar texture to said non-reflective focal conic texture,across said pixels to produce switched pixels; applying a fourth pixelvoltage, incapable of switching said pixels from said stable planartexture to said non-reflective focal conic texture, to produceunswitched pixels to remain in the stable planar texture; and repeatingsaid addressing until said rows of pixels of said area to be updatedhave been addressed; and in phase 4, removing said first pixel voltage,said second pixel voltage, said third pixel voltage, and said fourthpixel voltage from said area of said bistable matrix-addressable displayelement to be updated.
 18. The method of claim 17 wherein said firstpixel voltage, said second pixel voltage, said third pixel voltage, andsaid fourth pixel voltage comprise AC voltages.
 19. The method of claim17 wherein said addressing is sequential.
 20. The method of claim 17wherein at least one of said first pixel voltage, said second pixelvoltage, said third pixel voltage, and said fourth pixel voltage is avoltage pulse.
 21. The method of claim 17 wherein said bistable chiralnematic liquid crystal imaging layer comprises a polymer dispersedbistable chiral nematic liquid crystal imaging layer.